The movie contains approximately 1,547 digital effect shots. Worldwide FX produced 1,186 of the shots, and a further 800 shots which were discarded in response to changes to the film’s story. Digital effect pre- production began in July 2011 and was completed one year later.
Worldwide FX’s primary facility is situated in Sofia, allowing them to have constant access to the production, and model assets of the sets before their practical versions were built; this enabled Worldwide FX to raise issues relating to the designs early and develop an optimised construction plan. Anything that was considered to potentially need a computer generated double was photographed and digitally modeled even if it was not scheduled to appear in the film, which later served useful as the film’s story was adjusted, requiring Worldwide FX to rebuild entire scenes.
Several sets were rendered digitally, and almost all of the sets had some form of digital enhancement. The largest digital environment was the Devetashka cave, which had to be digitally recreated because of the environmental concerns of filming explosions in the real cave. Using over 5,000 photographs taken at different times of day, the cave was accurately illuminated and rendered down to crevices and moss. The cave’s interior containing a concealed nuclear vault was subjected to several changes up to two months before the effects’ delivery date based on editorial input, including modifications to its scale. The rail car used within the cave was also digitally built alongside a practical version, but the digital versions versatility saw it largely replace its practical counterpart. The scene featuring the Expendables and prisoners trapped in the cave were shot entirely on Chroma key with the cave background inserted later.
The entire opening sequence of the film was shot in Bulgaria during the winter, but was meant to take place in tropical South Asia. Digital effects were used to remove the snow and deciduous trees present and replace them with tropical flora. The alterations required over 340 shots alone. The scene was further complicated by Jet Li’s scheduling conflict, which meant the interior sets had to be shipped to Hong Kong, reassembled and shot with him there, and the exterior sets had to be digitally built and then merged with footage of Li shot on Chroma key. A fish tank in the opening was filled with digital eels based on South Asian river eels, but increased in size by three times, after the real fish died and sank to the bottom. The finale in the airport was another complex digital environment, shot in three entirely differently locations in two seasons, which were then merged to create one seamless location. When the trucks break through glass to exit the airport, the scene is entirely digital and was filmed on Chroma screens in the middle of Sofia. Numerous digital matte paintings were used to modify the winter Bulgarian terrain in different scenes to represent its intended location.
Vehicles also received digital enhancement. For the scene in which the Expendables crash their Canadair CL-215 amphibious plane into a cave, the effects team intended to use Miniature effects, filming using a 1/3 scale model of the 90-feet wing span plane. Production constraints and the limited shooting schedule meant the miniature shoot was abandoned and the entire sequence was rendered digitally. The plane itself was also digitally modified, adding a nose mounted gun which in practice could have stalled the engine with its recoil. A rear hatch door was required for jet skis to drive into the plane but the crafts design made such a door impractical; visual trickery was used to obscure this fact and make the door fit into the existing plane design. A helicopter used in the opening sequence was required to perform more dynamic moves and so it was digitally removed entirely and replaced with a digital recreation that better fit the scene’s requirements. The helicopter which decapitates Hector during his fight with Christmas had no rotors and they were digitally added. WWFX later enhanced it more, modifying lighting and stabilizing the helicopter’s body which shook unnaturally when struck during the fight. The Sangs Mi-8 helicopter was a civilian variant, but it was digitally replaced with the gunship variant, the Hip-E, to make it look meaner and more fitting for Van Damme’s villainous mercenary, with a black reflective coating and armaments. The T-72 tank used to assault the Expendables would have devastated the set with the resulting concussive wave if it had fired a single shot, so it was replaced in most shots with a digital version that performed the shooting.